"Hepaciviruses and pegiviruses are a group of RNA viruses, like lyssaviruses that infect a diverse array of animal species," says evolutionary biologist and virus specialist, Professor Edward Holmes, of the University of Sydney.

"This paper shows that in nature, there are a lot of them but in humans the only one we really know about that's a major health problem is hepatitis C virus."

Holmes says research into SARS showed the virus responsible ultimately came from bats, and this spurred a large amount of research into the viruses carried by bats.

"SARS was a real wake-up call for people working on emerging diseases," he says.

In the largest study of its kind, Holmes and colleagues used molecular techniques to investigate viruses in bats across the world.

PCR screening of nearly 1300 bat specimens identified 83 new hepaciviruses and pegiviruses, with 5 per cent of the bats being infected.

"Five per cent may sound low, but it's actually very, very high. That's the number that are actively replicating with virus at that time," says Holmes.

He and colleagues found that the diversity of the viruses harboured by bats was much greater than had ever been seen before.

Origins of hepatitis C

Holmes says even though hepatitis C is a member of hepacivirus genus, the relationship between hepatitis C and bats is unclear.

"Bats have clearly played a major role in their [hepaciviruses and pegiviruses] evolutionary history," says Holmes. "Where individual viruses, such as hepatitis C virus, came from no one knows."

To date, he says, the closest relative to hepatitis C is in dogs and horses, but stresses he and colleagues have not found any bat virus that is directly ancestral to human hepatitis C.

The study also found that hepaciviruses and pegiviruses have been in bats for a very long time.

"We now know the hepaciviruses and pegiviruses are ancient in bats," says Holmes.

"Paradoxically it tells us bats are not the threat we thought they were because we've been living with them for aeons."

Risk in perspective

Holmes stresses that even though bats are a huge reservoir of viruses this should not alarm people.

"People shouldn't get overly worried about bat viruses," he says. "In fact, very rarely do these viruses directly come to humans. Lyssavirus is one of the few examples where a bat directly causes a disease in humans."

He says in most other cases, bats must pass the virus on to another species that lives closely to humans, before the virus is passed to humans.

For example, says Holmes, in the case of SARS, the virus was passed from bats to civets and then to humans.

He says while the consequences of viruses such as this can be bad, it is low compared to the risk posed by influenza virus which does not come from bats.

"You've got to put the risk in context, and it ain't great," says Holmes.